In general, a silicon wafer (hereinafter, also simply referred to as a wafer) with a diameter of 300 mm or more is produced through the following producing processes. First, a silicon ingot is sliced into thin disk-shaped silicon wafers in a slicing process, and the outer region of the wafer obtained in the slicing process is chamfered in a chamfering process to prevent a fracture and a chip in the wafer. Then, the chamfered wafer is flattened in a lapping process, and mechanical damage remaining in the chamfered and lapped wafer is removed in an etching process. Then, the surface of the etched wafer is mirror-polished and flattened in a polishing process, and the polished wafer is cleaned in a cleaning process to remove a polishing slurry and foreign substances attached to the wafer.
The processes described above are only main processes, and other processes such as a heat treatment process and a surface grinding process may be further added or the sequence of processes may be changed. Moreover, the same process may be performed more than once. Then, an inspection or the like is performed, and the silicon wafer is sent to a device production process and an insulator film and a metal wiring line are formed on the surface of the silicon wafer, whereby a device such as a memory is produced.
Besides, the polishing process is generally a process of polishing and flattening the surface of a silicon wafer to a mirror-smooth state by bringing the silicon wafer into sliding contact with a polishing pad while supplying a polishing slurry. In the polishing process of the silicon wafer, usually, polishing is performed in multiple stages from rough polishing to finish polishing. In general, primary polishing is performed by double-side polishing, and then secondary polishing and further finish polishing are performed by single-side polishing to remove damage caused in the primary polishing and improve surface roughness.
In the double-side polishing, the silicon wafer is held in a holding hole of a carrier, and the carrier is interposed between upper and lower turn tables to which polishing pads are attached. Then, both surfaces of the wafer are brought into sliding contact with the polishing pads by rotating the upper and lower turn tables in opposite directions to each other while supplying a polishing slurry to the polishing pads, whereby the surfaces are simultaneously polished (for example, see Patent Document 1). Moreover, the double-side polishing often employs a method in which a plurality of silicon wafers are polished at the same time and the polishing is repeated in a batch manner.
In the single-side polishing, the silicon wafer is held with a polishing head, and one surface of the silicon wafer is brought into sliding contact with a polishing pad attached to a turn table by rotating both the turn table and the polishing head while supplying a polishing slurry to the polishing pad, whereby the surface is polished (for example, see Patent Document 2). The single-side polishing is often employed in the finish polishing process for improving surface roughness of the silicon wafer. The thickness of the silicon wafer to be removed (a polishing stock removal) in this finish polishing process is a little 0.1 μm or less. Moreover, in the finish polishing, disposable slurry is often used for polishing to prevent occurrence of a scratch or the like.
As the slurry used for the above-described polishing of the silicon wafer, a polishing agent in which fine SiO2 (silica) abrasive grains or CeO2 (ceria) abrasive grains are colloidally dispersed in an alkali aqueous solution with a pH of about 9 to 12 is used. Such slurry polishes the silicon wafer by a combination of a mechanical effect of SiO2 or CeO2 and a chemical effect of the alkali aqueous solution to etch silicon. However, silica abrasive grains or ceria abrasive grains contain metal impurities in a trace of amount. Examples of the metal impurities contained in polishing abrasive grains such as silica abrasive grains or ceria abrasive grains include nickel, chromium, iron, and copper.
In particular, metal impurities such as copper and nickel in the silica abrasive grains, which have lower ionization tendency than hydrogen, dissolve as metal ions in the alkali slurry, precipitate on the wafer surface during polishing of the silicon wafer, and deeply diffuse into silicon inside, thus degrading wafer quality and significantly lowering characteristics of a semiconductor device formed by the wafer.
To prevent the degradation of wafer quality due to the slurry containing polishing abrasive grains, for example, a polishing slurry containing highly purified silica abrasive grains is used. However, it is difficult to completely remove metal impurities in the silica abrasive grains. Then, there is disclosed a method in which a water-soluble chelating agent is added to a polishing slurry to trap metal ions by the chelating agent (for example, see Patent Document 3).
Therefore, there is a risk of increasing metal impurities unless the polishing abrasive grains and the water-soluble chelating agent are simultaneously supplied at a constant concentration ratio. Moreover, an amine, which is alkali, is often added as a polishing rate accelerator to the slurry containing polishing abrasive grains in order to improve the polishing rate.
However, since the alkali amine has high silicon etching rate, a remaining amine can locally etch the surface of the silicon wafer after completion of polishing, causing local roughness. This yields inferior products. Thus, after the polishing process with the slurry containing polishing abrasive grains and an alkali amine, post-processing is successively carried out with pure water to prevent the alkali amine from remaining on the surface of the silicon wafer.
In the finish polishing process, disposable polishing slurry is used to prevent occurrence of a scratch or the like, whereas in the polishing process prior to the finish polishing process, the used slurry is recycled to reduce the cost. More specifically, the used slurry is circulated and supplied to the silicon wafer for polishing.
As a technique for recycling the used slurry, for example, Patent Document 4 discloses a polishing slurry recycling apparatus. This polishing slurry recycling apparatus includes a slurry recovering means for recovering a slurry used for polishing, a recovered slurry guiding means for guiding the recovered slurry into a recovering tank, a new slurry supplying means for supplying a new slurry into the recovered slurry, a recycled slurry producing means for mixing the new slurry and the recovered slurry to produce a recycled slurry with a homogeneous concentration, and a measuring means for measuring the concentration of the recycled slurry produced by the recycled slurry producing means.
The new slurry supplying means supplies a new slurry with higher concentration than the recovered slurry and stops supplying the new slurry when the concentration of the recycled slurry measured with the measuring means exceeds a predetermined value of the concentration of the slurry to be supplied. The used slurry is circulated and supplied to the silicon wafer for polishing.